Vulcanization, or curing, is a process whereby elastomers, either natural and/or synthetic, are mixed with various materials which will cause the elastomer to undergo crosslinking upon application, of heat. These materials are conventionally compounded with the elastomer to help improve the elastomer's cured physical properties, e.g., tensile strength and temperature sensitivity. Vulcanization and the resulting improved properties may be obtained by reacting the raw elastomer with sulfur in the presence of other cure activators. Oleic and stearic acids have been commonly used as activators in sulfur vulcanization of diene rubbers in the presence of zinc oxide and an accelerator.
During the early stages of a typical vulcanization process, a zinc carboxylate (the reaction product of zinc oxide and the fatty acid) reacts with the accelerator to form a complex. A nucleophilic attack by the complex on the ring of orthorhombic sulfur results in the formation of a zinc perthiomercaptide complex. This zinc perthiomercaptide complex is believed to be the sulfurating agent responsible for the crosslinking of the elastomer's chains. The role of the fatty acid is believed to increase the solubility of zinc oxide and subsequent reactivity of the zinc perthiomercaptide complex.
Stearic acid is probably the most commonly used fatty acid for vulcanization. However stearic acid suffers from the disadvantages of a high bloom rate and a rather high cost. The high bloom rate, i.e., the high rate at which the stearic acid comes to or “blooms” to the surface of the compositions, causes some loss of adhesion, properties. Another disadvantage of stearic acid is its high titer (on the order of 55° C.). This necessitates an elaborate heating system for the storage tankage and delivery systems, or addition of the stearic acid from bags.
Therefore, there exists a need for a cheaper and abundant material which can be used as a substitute for oleic or stearic acid with concomitant improvement in adhesion properties of the rubber. Recently, the use of TOFA has been described as an activator in rubber vulcanization (see, e.g., U.S. Pat. Nos. 4,895,911 and 4,870,135). Even more recently, the monomeric distillates from clay-based dimerization of an unsaturated fatty acid has been described to promote vulcanization (see U.S. Pat. Nos. 5,756,619 and 6,072,009). The present invention provides another product for rubber compounding (tall oil heads) that has not previously been recognized for this purpose.